I had a friend recently pick up a Windows Phone 8 device, and shortly thereafter, she began appearing in my messaging list twice. She has her original thread that had been there from her old phone and then the new thread that appeared afterward. If I tap the contact name at the top of the thread, they both lead to the same contact in the people hub.
I'd noticed this had happened before, to another contact, as well.
Had anyone else encountered this? Does anyone know the cause or a way to remedy the issue?

I did, I found it could be related to some number mix-up where it switches from 04xxxxx to +61 4xxxxxx (+61 being the country code for AUS in my case) (or vice versa) in the contact, and so the phone decides it must be an entirely new number and makes a new thread for that number

i've had that happen with text messages for contacts that haven't had any recent changes, just new separate threads randomly spawned. i'm sure there was some trigger that caused it to happen, but i'm not sure what it could have been.